The present invention relates to magnetic bubble memories, and more particularly, to a magnetic bubble memory with reduced coil power consumption due to non-uniform winding of its Y drive field coil.
Information storage and retrieval in the conventional magnetic bubble memory devices is accomplished and measured by the presence or absence of magnetic bubble domains which are propagated and manipulated on a chip. Typically, the chip includes a plurality of spaced apart permalloy propagation elements which overlie a thin film of garnet. The maintenance and propagation of the magnetic bubble domain along the paths of the propagation elements is accomplished through the utilization of an in-plane rotating magnetic drive field (XY drive field) in the presence of a bias magnetic field (Z bias field). As the drive field rotates, the bubbles jump between adjacent propagation elements crossing the gap which exists therebetween.
The bubble memory chip is typically sandwiched between upper and lower permanent magnets which supply the Z bias field. The XY in-plane drive field can be supplied by several geometries of coils. The most popular arrangement is shown and described in an article entitled "Bubble Memories Come to the Boil" by Peter K. George and George Reyling, Jr., published in the Aug. 2, 1979 edition of ELECTRONICS magazine starting at page 99. As shown therein, it is conventional to mount the bubble chip in the center leg of a E-shaped chip carrier to facilitate assembly of the orthogonal XY drive field coils. Drive field circuitry provides the electrical current to the XY coils which is required to generate the necessary rotating field. Though it has been demonstrated that the bubbles themselves are capable of 500-kilohertz operation, a practical upper limit is probably 200-kilohertz. This limit is set by the power dissipation in the drive field coils due to both skin-effect losses in the coil windings and eddy-current losses in the metal package components. Because of this, various current-access approaches that would eliminate the coils have been proposed. However, these have yet to be proven.
In constructing XY coils to drive bubble memory chips, designers have strived for field uniformity over the entire chip area. One explanation for this is that drive field uniformity will insure uniform, predictable bubble propagation at every chip location when the permalloy propagation elements have a preselected configuration and positioning. A perfectly uniform drive field cannot be obtained due to practical construction limitations. However, it can be approached by precisely winding the XY coils and by centrally positioning the upper surface of the bubble chip precisely in the middle of the coils.
In the uniform field approach the X and Y coils generally extend over the entire surface area of the bubble chip and overlap its edges somewhat. Neither the X nor the Y coil has a gap therein, i.e. successive turns of each coil are immediately adjacent and each turn abuts against a preceding turn.
Conventional bubble memories which include uniformly wound XY drive field coils typically dissipate approximately 0.75 to approximately 1.0 watts in such coils during operation. This power is dissipated in the form of heat which raises the temperature of the environment in which the bubble memory must operate. If the temperature of the bubble memory rises too high, spurious bubbles will be generated and the memory will not operate properly. Assume, for example, that the proper maintenance and propagation of bubbles can occur so long as the temperature of the garnet film is between approximately 0.degree.-100.degree. C. The packaging which surrounds the bubble chip, the coil, and the permanent magnet tends to retain the heat generated by power dissipation. This causes the temperature of the bubble chip to increase. If the temperature of the ambient air is approximately 40.degree. C., then the maximum temperature rise within the package that can be tolerated would be 70.degree. C. It would therefore be desirable to reduce the amount of power dissipated in the drive field coils in order to lessen the amount of heat generated thereby and thus permit the bubble memory to operate in higher ambient temperature environments.
Another important design rule to consider is that if the power requirement of the drive field coils can be reduced, cost savings in the drive field circuitry can be achieved. Integrated drive circuitry is preferable over drive circuitry incorporating discrete components. This is because the former is cheaper to manufacture on a large scale and requires less space.
Presently in most bubble memory chip architectures that have been considered for manufacture, data is stored in a plurality of data storage loops located in the central or medial area of the chip. The active components of the chip, including the transfer gates, replicate gates, and bubble detectors, are located in peripheral edge areas of the chip. Typically, the active components require a drive field magnitude in order to operate which is considerably greater than that required for the storage loops. For example, the active components in a bubble memory typically require a drive field strength of approximately 45-50 Oersteds while the storage loops only require approximately 35 Oersteds. It is conventional to provide a uniform drive field, i.e. one having the same magnitude at all locations on the chip, which is strong enough to drive the active components. Thus the bubbles in the storage loops are "over driven" and power wastage results.